This invention relates to improved halogen-containing rubbers and to a method of manufacturing these improved rubbers.
More particularly, this invention relates to the use of aromatic amino acids, salts of aromatic amino acids, esters of aromatic amino acids, amino phenols, salts of hydroxy benzoic acid and other compounds to improve the heat aging properties of halogenated rubbers.
Still more particularly, this invention relates to use of the above-listed compounds to improve the heat aging properties of chloroprene rubbers.
Chloroprene rubbers, also known as polychloroprenes and neoprenes, comprise a family of halogenated synthetic rubbers made by the free radical initiated polymerization of chloroprene. Members of the chloroprene rubber family are known for their resistance to solvents and as having excellent high temperature stability.
Processes for curing (i.e., vulcanizing) chloroprene rubbers are well known in the art. For example, vulcanization of chloroprene rubber is generally accomplished under pressure and at elevated temperatures using metal oxides, such as magnesium oxide, zinc oxide, and lead oxide, alone or in combination. These oxides, especially zinc oxide, are thought to catalyze the vulcanization reaction. Also, they regulate scorch (i.e., premature vulcanization) and cure rate. Of the many oxide combinations possible, it is generally recognized that combinations of magnesium oxide and zinc oxide are preferred. This combination produces high vulcanizate quality, in terms of heat aging, scorch resistance, tensile strength and coloribility, without unduly lengthening cure rates. Lead oxides are often used to improve water resistance.
The use of accelerators in combination with oxides is also known. Commonly used accelerators include thiourea accelerators, such as ethylenethiourea, diethylthiourea and dibutylthiourea, alone or in combination with accelerators such as tetramethylthiuram disulfide, tetramethylthiuram monosulfide, di-ortho-tolyl-guanidine and benzothiazyl disulfide. An excellent summary of chloroprene rubber curing systems is "Curing Systems for Neoprene", by S. W. Schmitt, which is available as technical paper NP-300.1(Ra) from DuPont.
Aside from the curing of chloroprene rubbers, many additives are known in the art as chloroprene rubber processing aids. For example, stearic acid is used to minimize the sticking of mill rolls and as an internal stock lubricant; low molecular weight polyethylenes are used as mold release agents and to improve extrudability; clays and carbon black are used as reinforcing fillers; Octamine, which is a reaction product of diphenyl-amine and diisobutylene, is used as an antioxidant; and process oils are used as plasticizers. Additionally, sulfur is used as a vulcanizing aid, and N,N-m-phenylene dimaleimide is used as a curing aid. Processing aids and their uses are summarized in "Processing Aids for Chloroprene", by S. W. Schmitt, available from DuPont as technical bulletin N.P.-350.1.
One key rubber property is its tendency to become scorchy. Scorching, as used herein, is defined as premature vulcanization. Scorch is typically correlated to Mooney viscosity measurements taken before and after bin aging of unvulcanized chloroprene rubbers. For example, unvulcanized rubber may be aged at a designated temperature for a specified length of time. Mooney viscosity measurements, before and after bin aging, indicate whether scorching has occurred. Ideally, the Mooney viscosity of a rubber compound will not change over the course of bin aging.
Mooney viscosity values are typically measured in accordance with ASTM test method D1646.
A second key rubber property is its heat stability. The industry has long sought methods of improving the heat aging properties of rubbers. Heat aging relates to the deterioration of vulcanized rubber under high temperature aging conditions. Thus, rubber specimens having known physical properties are subjected to the deteriorating influence of high temperature for standard periods of time. The specimens are generally exposed to circulating hot air at atmospheric pressure. The properties typically measured to determine deterioration include modulus, tensile strength, ultimate elongation and hardness. Thus, changes in these properties indicate the undesirable effects of heat aging. Heat aging tests on chloroprene rubbers are typically conducted in accordance with ASTM test method D865.
Halogenated rubbers, especially chloroprene rubbers, having improved heat aging characteristics would constitute a notable advance in the art. Accordingly, one object of this invention is to offer halogenated rubbers, especially chloroprene rubbers, having improved heat aging qualities.
A further object of this invention to provide a method for manufacturing these improved rubbers.
These and other objects of this invention have been accomplished by the addition of designated compounds to standard halogenated rubber formulations. Thus, the inventor has discovered that, by incorporating additives selected from the group consisting of aromatic amino acids, salts of aromatic amino acids, esters of aromatic amino acids, amino phenols and salts of hydroxy benzoic acid, among others, into halogenated rubbers during compounding, vulcanized products are obtained which have surprising and unexpected heat aging characteristics. These improved rubbers also have acceptable scorch qualities.
This invention greatly expands the versatility of halogenated rubbers, especially chloroprene rubbers. In certain applications, the utility of vulcanized chloroprenes is limited by their heat stability. The additives of this invention improve heat aging characteristics, thereby opening new area of application to these treated rubbers.
U.S. Pat. No. 3,027,351 discloses a use of anthranilic acid as an antioxidant in styrene butadiene latexes. Its use as an agent to improve heat aging in halogenated rubbers is not disclosed or suggested. Halogen-containing rubbers are distinct from styrene butadiene rubbers in chemical structure, vulcanization mechanism and physical properties.